1. Field of the Invention
The present invention relates to a method of producing an optical disc of bonded type.
2. Description of the Related Art
An optical disc of bonded type such as a conventional optical video disc is produced by bonding a first substrate having a first information record surface and a second substrate having a second information record surface together by an adhesive or bonding agent, from which an information signal on the first information record surface is read by emitting a laser beam thereto through the first substrate and an information signal on the second information record surface is read by emitting a laser beam thereto through the second substrate. In case of, after reading one of the information record surfaces, reading the other of the information record surfaces, the optical disc is inverted or the optical head is moved to a side of the optical disc facing the other of the information record surfaces.
Therefore, there is an optical disc of bonded type, in which one of the information record surfaces is half transparent (half reflective) to partially reflect and partially transmit the laser beam incident thereto, and an intermediate layer between the information record surfaces is transparent, so that both of the information record surfaces can be reproduced by emitting the laser beam from just one side of the optical disc.
As a method of producing such an optical disc of bonded type, there is one method of coating a liquid resin of ultraviolet hardening type on the information record surface of one of the substrates, overlapping the substrates to each other and rotating the overlapped substrates so as to fill the space between the substrates with the liquid resin of ultraviolet hardening type, and then emitting the ultraviolet light thereto so as to bond the substrates together. There is another method of bonding the substrates by use of a transparent adhesive or bonding sheet.
However, in case of the former method, there is a problem that air bubbles are easily entrapped in the intermediate layer and it is difficult to uniform the thickness of the intermediate layer. In case of the latter method, although it is relatively easy to uniform the thickness of the intermediate layer, there is a problem that it is difficult to process the sheet in line with the size of the inner and outer diameters of the disc and the cost is rather high.